This invention relates to a novel method of producing a photographic element, such as a lithographic printing plate, which, may optionally be employed as either a positive working or a negative working element merely by altering the relevant exposure and developing parameters.
Heretofore lithographic printing plates were capable of use solely as a positive working or a negative working plate. In a negative plate system, a substrate was coating with a diazonium salt admixed in an oleophilic resin. Such a plate was then exposed through a photographic transparency and the light struck areas became highly insoluble. As a result of this solubility differential, the unexposed areas, which remain soluble in a developer are able to be washed away and the exposed areas on the plate remain adhered to the substrate. In a positive working system, an aromatic diazo oxide composition is applied to a substrate and exposed. In this type of system the exposed portion of the element is removed by a developer while the unexposed portion remains adhered to the substrate. It has not been previously possible to convert a negative plate type to operate positively or vice versa at least without elaborate treatment techniques which are not economically feasible. By the present invention a single plate may be made to operate either positively or negatively with development by tap water solely by alternating the exposure and developer parameters.